You're kidding right? Get the range down right, and two rockets gives you a set of buffs, every time. Aim for the lead buff, and launch in pairs, about 2K out. One set for every two rockets. 110's are fun as heck doing this.
Put 12 on a 262 and people would be massacring buff sets. Just with the rockets, I'd top out with 18 kills. I'd expect around 10 or so, each time, during peak hours.
R4M's were very small rockets with an explosive charge, with a trajectory similar to a MK108's 30mm round according to what I've read. It wasn't an airburst rocket like the WGr.21, however 1 or 2 hits from the weapon were enough to bring down a bomber.
They were generally used like a shotgun from 600-800 yards as Krusty suggested. Still extremely deadly weapons. And they didn't have the extremely heavy, draggy stovepipes that the WGr.21 requires.
I think the Me 262A-2 was capable of carrying a pair of WGr.21's, though...